Lymphocytes bearing receptors specific for autologous antigens must be silenced in order to prevent the development of autoimmunity. Studies using immunoglobulin transgenic mice in which all B cells are autoreactive indicate that cell interactions with low avidity antigens can lead to silencing by anergy, a condition wherein the cell survives for a period of time (t1/2~5days) and retains its ability to bind antigen, but is unresponsive to immunogenic stimulation. Although it seems likely that autoimmunity is be caused by the inadvertent reawakening of these anergic B cells, the importance of anergy in maintaining immunologic tolerance in normal animals is unknown. Furthermore it is unclear how the antigen unresponsiveness of anergic cells is maintained and what "signals" might restore their responsiveness leading to autoimmunity. We have recently defined a set of cell surface markers that appear to be uniquely expressed on anergic B cells, and can be used to enumerate and isolate anergic B cells in mice with a normal diverse B cell repertoire. Studies of this population, referred to as the An1 (anergic 1) compartment, indicate that in normal animals 30-50% of newly produced B cells are destined to become anergic. Towards an understanding of their role in autoimmunity, we propose in Aim 1 the elucidation of inhibitory signaling circuitry that maintains the antigen unresponsiveness of An1 cells. Studies proposed in aim 2 address the ability of bacterial infection to prompt departure of cells from the An1 population and restore their responsiveness to antigen. Finally in Aim 3 we propose the characterization of a population of An1-like cells found in lymph nodes, exploring their responsiveness to antigen and the possibility that they arise as a consequence of autoreactivity acquired by somatic mutation. The proposed studies will employ transgenic models of B cell anergy as well as An1 populations derived from normal mice. These will be used in conjunction with biochemical and molecular genetic approaches to define regulatory signaling circuitry, and analysis of the effect of "danger", survival and T helper signals on anergic cell function. The proposed studies should advance our understanding of the genesis of autoimmunity by defining circumstances in which immunologic tolerance is broken by infectious agents and by mutations that disable molecular regulatory mechanisms critical for maintenance of anergy.